When we look at the world around us, most of us see in color. If we see a red object, that means we are receiving waves of a certain length, a length which our brain processes as the color red. If we see something blue, then our eyes are receiving waves of a certain length which corresponds to the color blue in our minds. If we weren’t capable of seeing the little bit of light in the electromagnetic spectrum that we can see (the light we can see is collectively termed “visible light”), and instead could see something like ultra-violet light, the world would look very different to us: “many white objects would appear black, our windows would be as opaque as sheets of iron, and polished silver would resemble anthracite coal,” as Robert Williams Wood, the first to publish infra-red photographs, put.
[I’m getting a large part of the information I’m summarizing from “A New Departure in Photography,” by Wood, but some of the information I add to connect ideas through time comes from what I’ve learned from school or other places.]
The ultra-violet region consists of of waves of light which are shorter in length than the waves of visible light. The infra-red region contains light waves that are longer than waves of visible light.
“With a photographic plate rendered sensitive to the entire spectrum we could obtain an idea of how things would appear if our eyes were sensitive to regions other than the one adopted by nature, by excluding from the lens of the camera all rays other than the ones with which we wish to operate.”
That’s exactly what Wood did. Wishing to take an infra-red photograph, he placed various glasses before the lens of the camera, which removed the visible light and prevented it from passing through but allowed the invisible waves of light “beyond the red of the [electromagnetic] spectrum” to pass through.
In the document, Wood included his photographs, some taken with ultra-violet light only, others with infra-red light only. In photographs taken with UV light, there are no shadows. Objects and landscapes are darker and blurrier in the black-and-white photographs, with trees and grass shining lighter and the sky generally black. According to Wood, “the presence of the slightest amount of haze in the atmosphere causes an illumination of the sky. In every case there is more or less illumination near the horizon.”
Wood explained that the sky was blue because of the sunlight. Sunlight consists of light waves that travel to places such as Earth. As the light passes through the atmosphere, waves of a certain length are scattered throughout, and those waves (because of their length) are processed as blue in our eyes. That is why we see the sky as being blue.
Something else that Wood found interesting about UV light was that in photographs taken solely with UV light, windows were black and one could not see through them at all. They appeared as solid walls do to us.
In the pictures Wood took of the window and open door (these pictures appear on page 4 of the article source, which I listed at the bottom of this post), Wood saw that flowers which were white to you and me—meaning that they appeared white with visible light—were black in the photographs taken only with ultraviolet light. This made him experiment with another white substance, the Chinese white pigment. With that pure white pigment, he wrote “Chinese White appears Black in Ultra Violet Light!” on a magazine page (pictures are on document page 5). Then he photographed the magazine page twice, once with a lens that photographed using visual light, and once with a lens that photographed using UV light. Guess what? The second time, when he photographed using the UV light, the words appeared black.
This was the caption of these pictures:
“The left-hand picture was made by ordinary light, the right-hand one by ultra-violet light. The white pigment absorbs the ultra-violet, and consequently appears black.”
This made me stop and think. I don’t know much about color, or physics, or etc. I’m in the process of learning, and hopefully always will be. He didn’t outright say it, but he implied that stuff that appear black to us are things which absorb light, and that’s why we don’t see another color. My mom’s an artist, and she told me something like that once, that black wasn’t a color and that neither was white, because a black object was actually one that simply absorbed light (does my memory serve me well?).
Anyway, if this is correct, then the reason the ink was black in the photo is that the camera which received UV light only wasn’t getting any from the pigment. The pigment was absorbing the UV light which hit it. I’m pretty sure that the only way that people see is by receiving light that hits something else. That light carries information about the object it hits to the eye, right? So if the pigment didn’t appear to have any color (assuming black isn’t a color), the light hitting the pigment wasn’t being released through the camera lens…
That creates a bunch of other questions, such as why certain things absorb light so well.
Anyway, apparently this “showing up black in UV light photos” trait doesn’t apply to all things which are white to our eyes, because when giving an example of UV light photography’s uses, Wood said that one could locate Chinese white pigment scattered on a hill of snow. (Take a photograph using UV light alone, and the Chinese white pigment will look black, in contrast with the snow, which will presumably not turn black.)
Wood wondered if any new information about the moon could be gained if it was photographed with UV light, and when comparing two photographs taken with different light (visible and UV), the UV light photo showed a dark substance to the right of Aristarchus, a bright lunar crater. This dark substance was barely visible in the photo of the moon taken with visible light.
Wood thought that the substance had “either flowed out of the crater in the molten condition, or, what is more probable, has been driven out in the form of blasts of volcanic ash or vapor.” He began photographing different volcanic substances (which were present, at least, on Earth’s volcanoes) with visible light and UV light, and made some discoveries. He learned that there were some substances that the bright spots on the moon/ dark deposit could not be. Some had speculated that maybe it was sulfur that gave Aristarchus its white glow. However, when photographing sulfur with visible and UV light, Wood found that while sulfur was white in visible light, it was “jet black” when photographed with UV light. Since when photographing Aristarchus with both visible and UV light, the crater had had a white glow, the crater could not be mainly sulfur; if it was, the crater would look black in UV light. This was an indirect discovery, I think—don’t think his main goal (at least, the article written by him implies that his goal was discovering the nature of the substance to the right of the crater) was finding out the components of the crater.
Zinc sulfate (not to be confused with zinc oxide, known as “Chinese white” and which changed to black in UV light photography) was white in the photo taken with UV light, meaning the substance next to Aristarchus, which showed up black in the UV light photo, could not have been zinc sulfate.
The following photos were not taken by Wood, as Wood published his article with his pictures (which had no color, they were just black and white) in 1910. Anyway, if the source can be trusted, the following pictures are of a single flower, once photographed with visible light (left) and once with UV light (right).
Wood moved on to discuss his future plans. He thought that with finer instruments, better images of the moon in different types of light might be taken. He thought that more modifications to photographic devices might result in cameras that could take pictures with a small part of the UV light spectrum, as opposed to with all the light from the UV light spectrum.
Now’s a good time to tell you how he even managed set up his camera to take photos solely with UV light.
Now, things that Wood said in the article made me wonder if he believed that all light waves smaller than violet in visible light were UV light. Look at the following image showing the electromagnetic spectrum:
You can see the small section of color, right? It’s enlarged on the bottom. That is visible light. It is a relatively small section of the electromagnetic spectrum that is visible light, and therefore we can see only a little bit of the electromagnetic spectrum. In Wood’s time… well, I haven’t really looked much into the science of the time, I’m just starting to. But obviously when he published the article, scientific knowledge was not what it is today. And I think that it’s possible that he was not aware of x-rays and gamma rays…
You know, scratch that… he should’ve have known, as a physicist, because doing a quick web search tells me that x-rays and gamma rays were discovered before 1910. Did he know where x-rays and gamma rays were located on the electromagnetic spectrum? If so, then why did he appear to think that all light beyond the violet (all light with waves smaller than that corresponding to the color violet) was UV light?
Here is the bit he said which puzzles me:
“If we perform the experiment in a dark room, and place a photographic plate in such a position as to receive the image of the spectrum, we find that it extends to a considerable distance beyond the violet.”
Basically, the electromagnetic spectrum extends beyond the color violet of visible light.
“In other words, there is a region beyond the violet which the eye cannot see, but which the sensitive plate is able to photograph. This region is termed the ultra-violet, and the light from virtually all sources contains a greater or less amount of this ultra-violet light.”
Except that the region beyond the violet is not all termed the ultra-violet: that region is divided into ultra-violet light, x-rays, and gamma rays, as seen in the image above. This misunderstanding is significant in my opinion because he never addressed x-rays or gamma rays. What if, when he said he photographed using UV light alone, he meant that he photographed with a lens that took in light waves that were smaller than violet, but not necessarily just light waves of UV light? Then that might mean that his “photographs taken only with UV light” were actually photos taken with gamma rays, x-rays, and UV light waves.
My reasoning is sound, I think—given the information I have. I myself may very well be missing important information that might lead me to make an erroneous conclusion. Sorry for my ignorance if that is the case.
Anyway, let me show you what he did to take photos with UV light alone. (Again, I don’t know if his photos were taken solely with UV light, or with other light, too. I’m just back to summarizing what I read.) He had observed that UV light could not pass through glass. (Remember, that’s why windows didn’t “work” in UV light photos—they were opaque like solid walls because light didn’t pass through them and carry to our eyes an image of the outside. That’s how glass is see-through to us: visible light passes through glass, sending our eyes information about what it looks like beyond the glass.) Therefore, if he wanted to take pictures with UV light, he could not use a camera with a glass lens, because then all UV light would be stopped by the glass and wouldn’t pass through. Wood solved the problem by using a lens that was “cut from a crystal of quartz.” Great, now UV light could pass through the lens—but so could visible light. Somehow, he had to stop visible light from passing through while allowing UV light to pass through. Wood discovered that by placing a thin sheet of metallic silver over the lens, UV light could pass through it, but visible light could not. (Of course visible light can’t pass through metallic silver—if it could, we could see through it. It’s cool how it all makes sense, right?) This meant that thanks to the special lens, Wood could photograph using UV light only. (But like I said…)
Wood also said he would try an experiment in the future on white flowers, growing them under a glass to prevent them from receiving UV light and seeing if he could detect a difference between them and flowers that had been exposed to UV light. I just realized that I have a tiny plant indoors (it’s literally a leaf that’s about the size of my thumbnail with roots in water) that is exposed to the sunlight that shines through my window during the day. It isn’t exposed to UV light. I really wonder what the effects of this are.
Then, before terminating the article, he wrote about mirrors. He said that mirrors are practically useless in UV light, reflecting very little of the light which reaches them, if any.
“If our eyes were sensitive to the ultraviolet region only, we should have to make our mirrors of polished nickel or tin; for silver, which is the most powerful reflector known for visible light,—reflecting, when freshly polished, about ninety-six per cent, of the light which falls upon it,— is able to reflect only about four per cent, of the ultra-violet radiations with which we are concerned; in other words, it has about the same reflecting power as a sheet of ordinary window-glass for visible light.”
This article was published in 1910 in The Century Magazine. I read this article knowing some things may be incorrect, knowing that Wood’s information in the field was incomplete, and etc., but among my reasons for reading the article are understanding his work and the scientific knowledge of the time. Thanks for reading my paraphrase of it!